Improve periodic USB transfers (device intr, isoc) used for input

Improve periodic USB transfers (device intr, isoc) used for input
devices, audio, and video.  It's still not perfect, and will need
further improvements.

High level, the diff contains following changes:

* Sync up with the Linux code base, which did re-work the periodic
  scheduling code path.
* Run the driver in IPL_VM instead of IPL_USB to prioritize us before
  lower/equal interrupts (same what NetBSD does).
* Add two new flags to our USB stack required by the updated driver
  code:
  - 'multi' flag in the usbd_hub structure to keep track whether
    a hub has one Transaction Translator for all ports (single TT)
    or one Transaction Translator per port (multi TT).
  - 'hcpriv' pointer in the usbd_tt structure for the HC driver to
    allocate memory for the scheduling depending on single or multi TT.

"go for it" kettenis@